Automatic-transfer-switch guidelines

Many critical electrical systems are not being maintained to the proper standard, and some are not maintained at all.

Why? Maybe it's because electrical systems and components are being installed improperly. Maybe it's because of poor system design that leaves little access. Or maybe it's because many facility engineers still regard maintenance as a necessary evil. In order to evaluate the cost of maintenance, one must factor in the cost of lost service due to an unplanned failure plus the cost of replacement.

The automatic transfer switch (ATS) is a critical system component of the emergency power system, and proper maintenance of an ATS depends on the type of switch and its position in the critical power infrastructure.

There are four basic ATS types:

break-before-make (open transition)

make-before-break (closed)

delayed transition (center off)

solid state

The break-before-make (open transition) is the most common. As its name implies, the load will be interrupted during the transition from the normal to the emergency source.

The make-before-break (closed transition) allows a hot-to-hot transfer without the loss of critical load.

The delayed transition (center off) switch is especially suited to applications in which large inductive loads result in large inrush currents. The delayed transition allows magnetic fields to completely collapse before reconnection.

The solid-state ATS lacks the traditional mechanical transfer switch. Because this unit relies on SCR or transistor technology, sub-cycle transfers are possible. Although there is technically a "break" in load current, the speed of these units prevents adverse effects to sensitive equipment.

Some manufacturers of ATS units provide the option of an isolation bypass feature. This feature allows maintenance or repairs to be made with no impact to the critical load.

Regardless of manufacturer, the automatic transfer switch provides the following basic functions.

Upon sensing the loss of normal or street power, the ATS logic signals the emergency engine or turbine generator system to start. When the emergency source of power is available at the ATS, the control logic samples the source to ensure critical parameters are within proper tolerance. At this point, the ATS will transfer to the emergency power supply.

These steps are reversed upon the return of the normal power source. The ATS logic will again verify the street power is within the desired parameters and stable prior to retransfer. Once the ATS is back in the normal position, the ATS logic dictates a cool down period for the emergency power system. Note that most set points and timing functions are adjusted to the customer's requirements.

ATS preventive maintenance checklist The following is a list of the basic tasks required to maintain an ASCO ATS.

1. De-energize the switchgear (ATSs equipped with an isolation bypass feature do not need to be de-energized).

2. Remove the arc chutes and pole covers. Consult the manufacturer's information for proper procedure. This step will allow visual inspection of the main and arcing contacts.

3. Test and recalibrate all trip-sensing and time-delay functions in the switchgear. Depending on the manufacturer, the steps required here will vary. The focus here should be to verify and record what current settings are and to ensure the current adjustments meet the customer's needs and expectations. If adjustments are necessary, the means to make and verify those adjustments need to be examined. For example, a voltage pick-up or dropout adjustment may require the use of a variable source such as a variable ac transformer. The standby engine can be a source of variable frequency, etc. In any case, the manufacturer is your source for information concerning these adjustments.

4. Vacuum the accumulated dust from the switchgear and accessory panels. Never use air to blow out dirt. Subjecting the TS unit to compressed air may have a detrimental effect by forcing dirt and debris into the switch mechanism.

5. Inspect for moisture or signs of previous wetness or dripping.

6. Clean grime with an approved solvent. Consult the OEM for a recommendation.

7. Inspect all insulating parts for cracks or discoloration due to excessive heat. Part of any complete maintenance program is an infrared scan. This work is done prior to maintenance with normal loads applied to the gear being scanned. The resultant report will define problem areas. The use of this information will allow the maintenance provider to take a proactive approach.

8. Inspect all main arcing contacts for excessive erosion. Arcing contacts are intended to be sacrificial by nature. They take the brunt of the energy when making or breaking the load. Careful attention should be paid to these contacts.

9. Inspect all main current-carrying contacts for pitting and discoloration due to excessive heat.

10. Inspect all control relay contacts for excessive erosion and discoloration due to excessive heat.

11. Manually operate the main transfer movement to check proper contact alignment, deflection, gap, and wiping action.

12. Check all cable and control wire connections to the transfer switch control and sensing panel and other system components and tighten if necessary.

13. Re-energize the switchgear and conduct a test by simulating a normal source failure.

These listed tasks do not appear especially extraordinary; however, steps 3 and 11 require training and knowledge. More than one failure can be attributed to inattention or ignorance in these areas. Imagine a major data center crash because the UPS ran out of battery as a result of a maladjusted ATS control panel. The reality of the situation is that proper maintenance of all components of the emergency power system is essential and directly linked to the integrity of the critical power system. This system is only called upon to function in an emergency. Of course, this is the worst time for a malfunction of any one element. The total aspect of maintenance extends far beyond the routine tasks recommended to properly maintain any one element. Just as with the example of the ATS, maintenance programs must be carefully thought out.

The selection of a maintenance provider is just as important as any other aspect.

Look under the hood. A surface comparison of maintenance proposals is sometimes misleading. Ask any aggressive maintenance company how their program measures up and you will probably be told that they can do everything you ask. Go beyond that, ask the tough questions, and ask for references. Don't settle for the low bid. It's been said, "you get what you pay for." I prefer, "pay me now or pay me later."